Apparatus for plasma welding

ABSTRACT

Plasma welding of light-metal workpieces and materials subject to oxidation is carried out using square-wave alternating current. The energization circuit comprises a bridge of controllable rectifier-type elements, the welding electrode network being positioned in a diagonal of the bridge while a direct-current source with a declining characteristic is connected across the other diagonal.

Dauer et a].

APPARATUS FOR PLASMA WELDING 3,549,973 12/1970 Steams et a1. 219/131 R X2 1 Horst Pfaffenhofen; PM 31231333 311333 8221;? 2191141 111Hildebrand! lsmaning, both of 3,598,954 8/1971 Iceland .1 219 13: RGermany 3,382,345 5/1968 Nonnando 219/131 WR Assignee: Messer GriesheimGmbH,

Fran fu t/Ma m y Primary ExaminerR. F. Staubly d: S L 13 1971 AssistantExaminerGale R. Peterson 1 e ep Attorney-Karl F. Ross Appl No.: 179,896

Foreign Application Priority Data ABSTRACT Sept. 18, Germany P Plasmawelding of workpieces and materi als subject to oxidation is carried outusing square- 219/121 219/131 wave alternating current. The energizationcircuit d 131 WR comprises a bridge of controllable rectifier-type ele-0 121 P 323/123 ments, the welding electrode network being positioned ina diagonal of the bridge while a direct-current R f d source with adeclining characteristic is connected e erences I 6 across the otherdiagonal. UNITED STATES PATENTS 1,950 7/1964 Chiasson 219/108 X 8Claims, 5 Drawing Figures 1 D-CSOURCE VITH H 79 8 5 i T ER/STIC PLASMAFORMING GAS HIsH FREQUENCY SOUECE (PLASMA IGNITIGM) PLASMA A 2c PATENTEDDEB 2 5 i975 SHEU 1 BF 3 Horst Dauer Peter Hi ldebrandf INVENTORS.

$5 9 R9 Attorney fi l FIG:

sum 3 M3 I N VEN TORSZ Horsf Dauer Pefer Hildebrand? Attorney PATENTEDUECZ 5 i975 utbmm: u QM $6 @5253 ES 356% 0d l APPARATUS FOR PLASMAWELDING FIELD OF THE INVENTION Our present invention relates to anapparatus for the plasma welding of metals and, more particularly, to asystem (i.e. a method of and a circuit for plasma welding) whereby aplasma is used to melt light metals, materials in thin sheets and othersubstrates in which oxidation of the molten metal and/or burnthrough ofthe substrate is a problem.

BACKGROUND OF THE INVENTION As is generally recognized, a plasma is anionized stream of gas in a more or less stable condition, generally atsuch high temperatures that an ionized state is maintained in spite ofthe recombination of charged species in the stream. Plasma-arc welding,plasma cutting and other processes are known in which the hightemperature of the ionized stream, i.e. the plasma is employed to melt ametal. During such processes, the plasma may be generated continuouslyby feeding an ionizable gas through a plasma generator in which a nozzlereceives with close spacing a plasma electrode and an electric dischargeor are is maintained between this electrode and a surrounding portion ofthe nozzle wall. The gas thus traverses an electric discharge and isionized so that it can be projected at high temperatures against theworkpiece. For the most part, plasma welding, i.e. the fusion of metalsto metal using the high temperatures of an arc-generated plasma, isenergized by direct current. The use of direct currents have, however,been found to be disadvantageous in that, especially for the welding ofthin metal bodies, the plasma arc is relatively unstable, Especiallywith light metals, such as aluminum, the pool of metal created by theplasma torch develops an oxide layer which alters physical properties ofthe weld. Moreover, the development and destruction of the oxygen layervaries the contact resistance or passive resistance in the region inwhich it is formed and causes abrupt changes in the voltage across theplasma are which generally is struck in part between the workpiece andthe plasma electrode. These fluctuations in electrical parameters areaccompanied by changes in the current amplitude so that the weld seammay become uncontrollable and of reduced quality. For example,excessively high current amplitudes may result in burnthrough of thesubstrate and the formation of holes therein while relatively lowcurrentamplitudes may preclude effective bonding of the weld metal to asubstrate metal and vice versa. Thus it is important to prevent theformation of substances during welding which may interfere with aconstant current flow between the plasma torch and the substrate.

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved apparatus for the plasma welding of aworkpiece whereby the aforementioned disadvantages may be obviated.

It is another object of the invention to provide a system for plasmawelding a metallic workpiece, especially thin substrates composed oflight metals, which will prevent burn-through and nevertheless providean effective bond between the substrate metal and a weld metal.

Yet another object of the invention is the provision of an improvedapparatus for the plasma-arc welding of such materials and especially animproved circuit for controlling such welding which, at low cost,provides improved constant current performance with a plasmaweldingdevice.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter are attained, in accordance with the presentinvention, in a system for plasma welding a substrate in which theelectrode of the plasma torch is energized with a substantiallyrectangular waveform pulses of alternating current, especially anelectric current so applied that the workpiece is briefly brought to anegative potential and the plasma electrode to a positive potential torestrict the formation of an oxide layer.

In other words the invention resides in a method of and a circuit forthe operation of a plasma torch or burner in which the workpiece and theelectrode of the plasma torch are alternately poled negatively andpositively, respectively, during one half cycle of operation and shortlythereafter are reversed in polarity, thereby preventing the formation ofoxide films, even when the plasma torch is used for the cutting andwelding of socalled light metals and relatively thin workpieces. Thesystem of the present invention has also been found to stabilize theplasma arc, even when the arc is contracted. We have already indicatedthat the invention finds its principal utility in processes involvingthe welding of thin sheets of light metals, especially aluminum andtitanium. It has also been found to be effective for the welding of thinsheets of zirconium which has, in the past, posed particular problems.

The square-wave pulses, according to the present invention, are providedpreferably with a slight spacing or interval and we define the intervalbetween the application of reverse pulses to the respective electrodes,i.e. the interval between switchover of the electrode or workpiece froma positive to a negative potential, or vice versa, as the switchovertime, the potential being brought to a zero or null value during thisperiod. Since the current flow at this interval is zero, there is atendency for the plasma arc to extinguish or quench and we prefer todimension this interval so that it is smaller than the deionizationperiod of the plasma are. In other words, this feature of the inventionis based upon our finding that, upon termination of the welding currentand welding voltage, the plasma arc extinguishes but only after a finiteperiod, i.e. the quenching is not instantaneous. We thus prefer todimension the polarityreversal time switchover interval so that it isshorter than this deionization period or the quenching time of theplasma arc. In this manner polarity reversal can be effected withoutdanger of extinguishing the plasma arc.

According to still another feature of the invention, the square wavecadence applied across the plasma generator and the workpiece is equalto the supply frequency of an alternating-current source, therebyeliminating the need for further circuitry to control the frequency ofthe periodically reversing pulses applied across the electrode and theworkpiece. However, when supply-frequency control is not used, asuitable oscillator operating at any frequency but preferably of theorder of magnitude of the supply frequency (50 to Hz), may be used. Thesupply-frequency control method and circuit, according to the presentinvention, simplifies and reduces the cost of the operation.

According to the instant invention, moreover, the welding current can beconsidered a train of rectangular current pulses in which a positivepulse over one half cycle is followed by a negative pulse over the nexthalf cycle. To increase the stability of the plasma arc, we have foundit to be desirable to maintain a ratio between the lengths (durations)of two successive rectangular-pulse half cycles of the welding currentin a ratio between 1:1 and 1:2.5 and to make these durations or lengthsadjustable. Preferably, the amplitude of the positive and negativehalf-cycle square-wave pulses are independently adjustable.

While we have used the term square wave herein in its commonly employedsense to mean a pulse having substantially vertical leading and trailingflanks and a constant amplitude over the duration of the pulse, thesignal may not accurately represent a square and in fact may berectangular. Hence, we use the term square wave in the broader sense ofmeaning rectangular pulses and may employ the term rectangular-pulsetrain to identify a similar sequence of pulses.

Best results are obtained when the rectangular pulse train of pulses ofalternating polarity are applied to the welding system so that over onehalf cycle, the welding voltage is relatively low and the weldingcurrent is relatively high. The welding current half-cycle pulses duringthis period can be considered good-burning pulses. The alternate pulses(i.e. those of alternate-half cycles) may then be termed oxide-limitingor oxide-controlling pulses or half cycles. The resulting plasma arc isfound to be especially stable. The good-burning half cycles generallyhave the workpiece poled positively to the electrode, i.e. the circuitapplies a negative pulse to the electrode. When the positive half cycleis applied to the electrode, the low-burning half cycle is encountered.When, on the other hand, the system is operated in the reverse sense sothat the applied potential is relatively high and the welding currentrelatively low, the welding operation becomes unstable.

We have found that it is desirable to maintain the stability of theplasma are by providing at the beginning of each low-burning pulse, i.e.a pulse in which a high negative potential is applied to the electrodeand high voltage but low current may be expected, an additional voltagepeak for a brief period to ignite plasma arc and prevent its extinction.When the welding current is a train of rectangular pulses in the cadenceof the supply frequency, e.g. 50 Hz, the half cycles may have durationsof about msec. The voltage peak may then be provided for a period of lto 2 msec. at the beginning of each low-burning pulse. With 6OI-lzsupply frequency a similar duration of the voltage peak may be employed.

To produce the rectangular alternating-polarity pulse train and tomaintain the switching time or polarityreversal interval at a minimum,it has been found to be advantageous to energize the plasma generatorwith direct current from a d-c source with a falling voltage/- currentcharacteristic whose time constant at most is equal to the polarityreversal time of the alternating current.

According to the apparatus aspects of the invention, the circuit forenergizing the plasma generator and applying the rectangularreversing-polarity pulse train across the electrode and the workpiececomprises four controllable diode-like or rectifier elements in a bridgecircuit, the electrode and workpiece being connected in series along onediagonal of the bridge while the other diagonal is directed across thedirect-current source with declining characteristics defined above. Theconcept of a controllable rectifier or diode-like element has beenintroduced to describe semiconductor devices having rectifiercharacteristics in the sense that they are conductive in only onedirection, but conduct only when properly biased or triggered. Many suchdevices have been provided in recent years and these include diodeshaving adjustable breakdown levels, transistors whose emitter-collectorelectrodes form a rectifying path, solid state controlled rectifiers andthyristors. In all cases, however, the unidirectional conductivity isrequired. The controllable diodes are preferably transistors orthyristors as described above.

The switchable rectifier devices of the bridge circuit are preferablytriggered in pairs through a multivibrator circuit or flip-flop operatedin the cadence of the supply frequency. To this end, we may providefullwave rectifier means for generating a train of rawrectifieddirect-current pulses each corresponding to a half cycle of thesinusoidal alternating current and a switch means, e.g. a bilateralswitch of adjustable breakdown potential, for producing a trigger pulsefor the multivibrator circuitry when the potential rises to apredetermined level during each of these directcurrent pulses.Preferably, the flip-flops are each provided with a transformerconnected in circuit to the bilateral switch and have outputs, dependingupon the state desired in the respective transistor, which are appliedthrough respective amplifiers to the transistors. Each multivibrator mayhave a second input for preventing false pulses from being generated asa result of signal noise or perturbations. The additional inputs to themultivibrators may derive from another bilateral switch and araw-rectified d-c source.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of the present invetion will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is a circuit diagram of a system for energizing a plasma-weldingarrangement according to the invention, somewhat simplified and inblock-diagram form;

FIG. 2 is a circuit diagram representing a detail of FIG. ll;

FIG. 3 is a circuit diagram showing other aspects of the invention;

FIG. 4 is a graph illustrating various wave forms appearing in thecircuit of the present invention; and

FIG. 5 is a circuit diagram representing the overall system and partlyin block form with other parts shown in idealized construction.

SPECIFIC DESCRIPTION In FIG. 1 of the drawing, we have shown fourtransistors I, 2, 3, 4 representing controllable rectifier elements ordiodes, whose collector-emitter networks are connected in a bridgehaving diagonals with terminals d; and d and diagonals ri -drespectively. One diagonal of the bridge is spanned by the electrode 5and the workpiece 6, forming the main welding-current network, while theother diagonal is connected across a direct-current source 8 and a choke7 with a sinteredflop 11. In a similar manner, the base of transistorll,

whose collector is connected with the choke 7 and whose emitter isconnected with the workpiece, can be biased into conductivity by afurther amplifier connected to the output 20 of a flip-flop 11 (see FIG.5). The transistor 2, which has its collector tied to the workpiece 6and its emitter connected to the negative terminal of the d-c source 8and the transmitter '3, which has its collector connected to the choke 7and its emitter connected with the electrode 5, are provided withrespective base circuits including amplifiers MI, these amplifiers 40,however, are energized by the outputs 21 of the respective flip-flop II. The circuit of the present invention thus provides each of the fourtransistors 1 4 with a respective amplifier 40 and a respectiveflip-flop 11, the opposite transistors being conductive simultaneouslyto pass respective signal pulses through the working diagoanl of thebridge. In all, four amplifiers 40 and four flip-flop 11 are employed asbest seen in FIG. 5. The flip-flop may be constructed as described atpages 362 ff. of PULSE, DIGITAL, AND SWITCHING WAVEFORMS, Millman andTaub, McGraw-Hill Book Company, New York, I965. The transistors of thebridge are of the silicon NPN type.

In FIG. 2, we have shown in somewhat more detail the contruction of theamplifier circuit represented at 40 in FIGS. 1 and 5. While theamplifier is shown here to be connected to the base of transistor 4, itshould be understood that similar constructions are used for the otheramplifiers which are connected to the respective transistors. Theamplifier 40 comprises two PNP transistors 9 and 10 whose output isfound at the collector of transistor 9 while the input is at the base oftransistor 10. The collector of this transistor is, as is generally thecase with cascade amplifier transistors, applied to the base of theoutput transistor 9. The input of amplifier 40 is connected with theoutput of bistable multivibrator 11 which has, as is customary, anotheroutput 21 of complementary state. For the transistors 2 and 3, theoutput 21 of the respective bistable multivibrators III are applied tothe amplifier inputs 40 (see FIG. 5). Furthermore, each of theamplifiers 40 has a emitter follower input which is represented as adirect-current voltage applied at 41. From FIG. 5, it can be seen thatthe emitter follower potentials may derive from transformer secondaries32 35 through rectifier bridges 32a 35a, energized via adjustableresistors 32b 35b. Each of the bistable multivibrators or flip-flopsllll has an input 18 from the secondary of a transformer 12, as

shown in somewhat more detail in FIGS. 3 and 5, and a further input 19derived from respective transformers 26, 36, 37 and 38.

From FIGS. 3 and 5, it can be seen that one input 18 of each flip-flopll derives from the secondary of a transformer 12 (22, 23, 24) whoseprimary is energized by a resistor 140 through a bilateral switch H4.The hilateral switch 14 may, of course, be a diac which is triggered ata predetermined threshold or regulated by the reverse bias appliedthereto. The input of the bilateral switch I4 derives from a full-waverectifier bridge 16 forming a d-c source and bridged by a smoothingcapacitor IS. The bridge 16 is energized by the secondary 17 of anisolating transformer 39 at the supply frequency through a variableresistor 13.

The second input 19 to each of the bistable multivibrators ll derivesfrom the secondary of a transformer 26 (36, 37, 38 for the otherbistable multivibrators), whose primaries lie in series with acurrent-limiting resistor 27 and another bilateral switch 28 whose inputreceives pulsating direct current (raw-rectified or halfwave pulses)from a rectifier diode 30 and a transformer secondary 31 at the cadenceof the supply frequency. Capacitor 29 provides some smoothing of the d-cvoltage between the diode 30 and the bilateral switch 28.

The transformer 39 is also provided with secondary windings 32 35 which,via the resistors 32b 3512, the rectifiers 32a 35a, and conductors 4ll,supply the d-c potential to the four emitter-follower amplifiers 40.

The welding system is shown in somewhat more detail in FIG. 5, fromwhich it can be seen that the electrode 5 is surrounded by a gas tube 5aforming a plasma generator with the electrode and receiving aplasmaforming gas in the usual manner. A high-frequency source 5b may bemomentarily connected by a switch 5c across the electrode 5 and thehousing 5a to initiate the discharge forming the plasma, the are beingthereafter generated between the electrode 5 and the workpiece 6.

When the transistor pair 1, 4 is triggered into a conductive state bythe corrresponding signal from the respective flip-flop and thetransistor pair 2, 3 are in blocking condition, the negative pole ofsource 8 is connected to the workpiece 6 while the positive pole of thed-c source is applied to the electrode 5. Between the welding electrode5 and the workpiece 6 there can be measured an open circuit voltage of,for example, V. When the transformer 39 is operated with alternatingcurrents of a frequency of 50 Hz, the multivibrators are switched atthis cadence so that transistor pair 2, 3 are rendered conductive andtransistor pair 1, 4 are blocked after an interval of 10 msec. In thiscase, the workpiece 6 is poled positively with respect to the electrode5 for a period of another 10 msec., whereby the polarity is reversed.This open circuit-pulse train is represented in graph g of FIG. 4.

Upon ignition of the plasma arc, for example, by the application of highfrequency in the manner described, the open-circuit voltage is reducedto the working voltage of about 30 V with a corresponding increase inthe amplitude of the welding current. The level of the welding currentduring each pulse is determined by the characteristics of the d-c source8.

The control of the four flip-flops, described for the flip-flop ill ofFIG. 3, is effected as follows: The bilateral switch 14 is subjected toa pulsing raw-rectified direct current (waveform a of FIG. 4) having amaximum amplitude of, say, 24 V. Each of these pulses has a duration ofa half-cycle and thus about 10 msec. At a preset threshold of, say, 7 V,the bilateral switch 14 is triggered into the conductive state andremains therein until the applied voltage (waveform a) returns to 0. Ateach 10 msec., therefore, a new pulse is applied to the transformer 12and is in the form of the square-wave signal of waveform c of FIG. 4applied to the flip-flop I it. As a result, the output 20 of theflip-flop Ill generates a pulse train, as shown for waveform e of FIG.4. This pulse train represents the rectangular positive pulse train of aduration of slightly less than 10 msec.

and at a cadence with a period of 10 msec. which is applied to the arcgap. At the complementary output 21 of the multivibrator, a negativepulse train f of FIG. 4 of identical periodicity and pulse length isgenerated. By adjustment of the resistor 13, the threshold amplitude ofswitch 14 can be changed to increase the voltage rise by half-cycle orreduce it and enable the threshold voltage to be reached sooner orlater. The duration of the pulses produced by the flip-flop can thus beadjusted.

To prevent spurious signals from being generated at the flip-flop, each.of the flip-flops is provided with a further input at 19 designed toset the outputs 20 and 21 as previously described at the designedlevels. Thus, one transistor pair 1, 4, can be cut out while the othertransistor pair 2, 3 can be rendered conductive. The additional impulseis produced by the bilateral switch 24 and follows the half-waverectified waveform b in FIG. 4 at 20 msec. intervals as described forthe full-wave waveform a. The waveform b is so selected as to .producethe pulses (waveform a) slightly behind each of the pulses of waveformc. The waveform h of FIG. 4 thus represents the signal which is appliedto the electrode and it can be seen that each positive pulse includes anigniting voltage rise (overall) of, say, 150 volts, including theopen-circuit voltage of 100 V superimposed upon the machining voltagewhich falls to V during the process. Waveform 3 represents thesubstantially constant welding voltage during opencircuit operation.

We claim:

1. A plasma-welding system, comprising: four controllable semiconductordevices connected in a bridge support with each of the devicesconstituting a branch of the bridge;

a direct-current source connected in a first diagonal of the bridge andconstituting the sole weldingcurrent source;

a plasma-welding torch having an electrode spacedly juxtaposed with aworkpiece, said electrode and said workpiece being connected across saidbridge as the second diagonal thereof; and

a trigger network connected to said semiconductor device for selectivelyrendering same conductive to apply a rectangular wave form alternatingcurrent across said electrode and said workpiece.

2. The plasma-welding system defined in claim 1 wherein each of saiddevices is a three terminal solidstate element having a pair ofprincipal electrodes constituting the respective branch of said bridge,and control terminals connected to said trigger network, said triggernetwork including flip-flops for energizing pairs of control terminalsin alternate states and amplifier means between said flip-flops and therespective control terminals.

3. The plasma-welding system defined in claim 2 wherein each flip-flopincludes at least one bistable monovibrator having an input, means forproducing a direct-current pulse train of a predetermined cadence of abilateral switch interposed between the last mentioned means and saidbistable monovibrator and conductive upon a voltage rise during eachpulse of said train for activating the bistable monovibrator.

4. The plasma-welding system defined in claim 3 further comprisinganother bilateral switch movement connected to another input of saidbistable monovibrator and triggerable in response to a supply frequency.

5. The plasma-welding system defined in claim 4, further comprising atransformer having a secondary winding connected to each of said inputsand a primary winding energized by the respective bilateral switch.

6. The plasma-welding system defined in claim 5 further comprising arectifier diode connected in series with said other bistabile switch toa source of alternating current, and a capacitor connected between saiddiode and said other bistable switch.

7. The plasma-welding system defined in claim 1 wherein each of saiddevices is a transistor having its collector-emitter network forming therespective branch of said bridge, and a base connected to said triggernetwork.

8. The plasma-welding system defined in claim 7 further comprising asintered-iron-core choke connected between said source and said bridgein said first diago-

1. A plasma-welding system, comprising: four controllable semiconductordevices connected in a bridge support with each of The devicesconstituting a branch of the bridge; a direct-current source connectedin a first diagonal of the bridge and constituting the solewelding-current source; a plasma-welding torch having an electrodespacedly juxtaposed with a workpiece, said electrode and said workpiecebeing connected across said bridge as the second diagonal thereof; and atrigger network connected to said semiconductor device for selectivelyrendering same conductive to apply a rectangular wave form alternatingcurrent across said electrode and said workpiece.
 2. The plasma-weldingsystem defined in claim 1 wherein each of said devices is a threeterminal solid-state element having a pair of principal electrodesconstituting the respective branch of said bridge, and control terminalsconnected to said trigger network, said trigger network includingflip-flops for energizing pairs of control terminals in alternate statesand amplifier means between said flip-flops and the respective controlterminals.
 3. The plasma-welding system defined in claim 2 wherein eachflip-flop includes at least one bistable monovibrator having an input,means for producing a direct-current pulse train of a predeterminedcadence of a bilateral switch interposed between the last mentionedmeans and said bistable monovibrator and conductive upon a voltage riseduring each pulse of said train for activating the bistablemonovibrator.
 4. The plasma-welding system defined in claim 3 furthercomprising another bilateral switch movement connected to another inputof said bistable monovibrator and triggerable in response to a supplyfrequency.
 5. The plasma-welding system defined in claim 4, furthercomprising a transformer having a secondary winding connected to each ofsaid inputs and a primary winding energized by the respective bilateralswitch.
 6. The plasma-welding system defined in claim 5 furthercomprising a rectifier diode connected in series with said otherbistabile switch to a source of alternating current, and a capacitorconnected between said diode and said other bistable switch.
 7. Theplasma-welding system defined in claim 1 wherein each of said devices isa transistor having its collector-emitter network forming the respectivebranch of said bridge, and a base connected to said trigger network. 8.The plasma-welding system defined in claim 7 further comprising asintered-iron-core choke connected between said source and said bridgein said first diagonal.